Why can't I use column aliases in the next SELECT expression?

Question

Can I modify the next to use the column aliases avg_time and cnt in an expression ROUND(avg_time * cnt, 2)?

SELECT 
           


        

Answer 1

Aliases are not available until the virtual relation is actually created, if you want to do additional expressions using the aliases themselves you will have to create the virtual relation using as sub-query than run an additional query on top of it. So I would modify your query to the following:

SELECT stddev_time, max_time, avg_time, min_time, ROUND(avg_time * cnt, 2) as slowdown, path FROM
(
SELECT 
    COALESCE(ROUND(stddev_samp(time), 2), 0) as stddev_time, 
    MAX(time) as max_time, 
    ROUND(AVG(time), 2) as avg_time, 
    MIN(time) as min_time, 
    COUNT(path) as cnt, 
    ROUND(AVG(time) * COUNT(path), 2) as slowdown, path
FROM 
    loadtime
GROUP BY
    path
ORDER BY
    avg_time DESC
LIMIT 10;
)

I want to add here the reason your second query worked is because the query planner recognized those columns as defined directly in the table you're querying them from.